It is interesting that the original marketing plan pitched micro-expansions that look a lot like the DLC of today.
The expansions seem to serve as a marketing tool and collectors item rather than a cash grab, but I do wonder why there was no followthrough.
Possibly because this posed the expansions as floppy disks costing $4.95 – by the time Diablo was released it was on CD, and presumably the art required for expansions was much larger than a CD, and CDs were not as cheap as a single floppy disk.